1. Field of the Invention
The present invention relates generally to a method and system for controlling an automotive suspension system so as to achieve both riding comfort and vehicle driving stability. More specifically, the invention relates to a method and system for controlling the automotive suspension system depending upon various suspension control parameters, such as roll intensity, pitch intensity and so forth, with variable control characteristics. In more detail, the invention relates to a suspension control for controlling the automotive suspension system with control characteristics that vary with road surface conditions.
2. Description of the Prior Art
Recently, suspension control systems controlling the performance of vehicular suspension systems have been used to provide better riding comfort and better vehicle driving ability. Such suspension control systems control the damping characteristics or shock absorbing characteristics of the vehicular suspension system depending upon various vehicle driving parameters affecting riding comfort and drivability of the vehicle. For example, suspension control systems may adjust suspension characteristics depending upon road condition, vehicle speed, acceleration and deceleration of the vehicle, vehicular roll and so forth. In order to obtain desired suspension control characteristics. the suspension control system receives preselected vehicle driving parameters, compares a value representative of the preselected vehicle driving parameters with a Corresponding reference value and adjusts suspension characteristics accordingly.
For example, the co-pending U.S. patent application Ser. No. 678,806, now U.S. Pat. No. 4,652,010, filed on Dec. 6. 1984, the corresponding European Patent Application of which has been published as First Publication No. 01 45 013, discloses a vehicular roll suppressive suspension control system, which invention has been assigned to the assignee of the present invention. In the disclosed system, the stiffness of a vehicular suspension system is selected in accordance with vehicle speed and the rate of steering adjustment, which influence the centrifugal force on a turning vehicle. Shock absorbers in the suspension system can operate in either high or low damping force modes. Vehicle speed and steering wheel position are monitored, and the latter property is checked periodically to derive a measure of steering wheel acceleration. When steering wheel acceleration, i.e. the second derivative of the steering angle with respect to time, exceeds a threshold value, then shock absorbers are operated in the high damping force mode otherwise, they operate in the low damping force mode. The threshold value to which the steering wheel acceleration value is compared is derived as a function of vehicle speed.
On the other hand, the co-pending U.S. patent application Ser. No. 649,484 now abandoned, filed on Sept. 11, 1984, the corresponding European Patent Application of which has been published as First Publication No. 01 35 902, discloses a nose-dive suppressive suspension control system, which invention is also assigned to the assignee of the present invention. In the disclosed system, an automotive vehicle suspension has a damper with variable damping characteristics, specifically adjustable damping force. A controller responds to application and release of vehicle brakes by increasing the damping force. Increasing the damping force at the beginning of braking suppresses nose-dive of the vehicle. On the other hand, increasing the damping force at the end of braking suppresses pitching motion of the vehicle. This greatly improves the behavior of the vehicle sprung mass during braking. A similar nose dive suppressive suspension control system active in response to braking of the vehicle has been disclosed in the Japanese Patent First Publication (Tokkai) Showa 60-148710, published on Aug. 6, 1985. Furthermore, the co-pending U.S. patent application Ser. No. 744,380, now U.S. Pat. No. 4,733,883, filed on June 13, 1985, which has been assigned to the assignee of the present invention, discloses a suspension control system which can precisely control the damping characteristics of vehicular suspension in order to successfully and satisfactorily prevent dive and rebound. The suspension control system is designed to detect vehicle deceleration on the basis of variations in vehicle speed and to compare the vehicle speed with a predetermined vehicle speed criterion across which damping characteristics of the vehicular suspension varies between harder suspension mode and softer suspension mode. The vehicle speed criterion varies with vehicle deceleration so that the criterion becomes higher when deceleration is greater than a predetermined level. By hardening the suspension earlier during relatively abrupt deceleration of the vehicle, the intensity of nose dive is reduced and thus the intensity of rebounding motion after nose-dive can be reduced. This ensures riding comfort by reducing the intensity of pitching behavior of the vehicle front end. On the other hand, by delaying the hardening of the suspension during gentle deceleration of the vehicle, softer damping characteristics can be maintained for a longer period to ensure riding comfort by successfully absorbing bounding and rebounding shocks from the road surface.
A similar acceleration and deceleration dependent suspension control system has been disclosed in the Japanese Patent First Publication (Tokkai) Showa 60-154906, published on Aug. 14, 1985. Vehiclular acceleration and deceleration are detected based on fuel supply control pulse width or fuel injection pulse width which is taken as engine speed indicative parameter and vehicle speed.
The co-pending U.S. patent application Ser. No. 744,379 now abandoned, filed on June 13, 1985, corresponding European Patent Application of which has been published as the First publication No. 01 67 159, discloses a suspension control system which is variable of damping characteristics of the suspension depending upon road surface condition and which has been assigned to the common assignee to the present invention. In the disclosure, the suspension control system includes a road surface sensor which produces a road condition indicative signal. A controller detects bounding and rebounding motions of the vehicle suspension on the basis of the road condition indicative signal from the road sensor. The controller monitors when the bounding and rebounding motion exceeds a predetermined level and when its frequency falls within a given frequency range. These conditions reflect typical vehicle body behavior during travel along an undulating road, in which case, the controller produces a control signal to stiffen the suspension system. This suppresses pitching motion of the vehicle on undulating road so as to satisfactorily and successfully provide good riding comfort.
Furthermore. the assignee of the present invention also assigned the U.S. Pat. No. 4,600,215, issued on July 15, 1986. The invention disclosed in this U.S. Patent adjusts suspension control characteristics relative to road surface condition depending upon vehicle speed. For instance, a suspension control system for automotive vehicles automatically adjusts the damping strength of variable shock absorbers or other dampers in accordance with road surface conditions. The sensor signal reflects vertical displacement of the vehicle body from the road surface. The sensor signal is compared to a corresponding reference level which is variable depending upon a vehicle speed. The results of comparison give an indication of the degree and scale of irregularities in the road surface. The comparison information is sent to a suspension system controller which causes actuation of the shock or other dampers absorbers to a stiffer mode of operation when road surface roughness requiring harder suspension is recognized.
In addition the co-pending U.S. patent application Ser. No. 791,766, now U.S. Pat. No. 4,616,848, filed on Sept. 3, 1985, discloses a suspension control system includes a suspension system variable of stiffness and/or damping characteristics, a road sensor producing a road condition indicative signal and a controller for controlling the stiffness and/or damping characteristics of the suspension system depending upon the road condition indicative signal value and the vehicle speed indicative signal value. The controller is detective of the vehicle speed in a predetermined normal speed range which is defined by a lower speed criteria and an upper speed criteria, for generally maintaining the stiffness or damping characteristics of the suspension in relatively soft. The controller is detective of the vehicle speed indicative signal value smaller than the foregoing lower speed criteria to increase sensitivity of the road sensor and/or harden the suspension system at harder level than that in the normal speed range when the roughness of the road surface beyond a rough road criteria. Also, the controller is detective of the vehicle speed indicative signal value greater than the upper speed criteria for increasing sensitivity of the road sensor and/or harden the suspension system at harder level than that in the normal speed range when the roughness of the road surface beyond a rough road criteria, the relative displacement between a vehicle body and a road wheel, a vehicle speed sensor producing a vehicle speed indicative signal.